It is known from K. Hyeongnam, N. Digbijoy, R. Siddharth and L. Wu: Polarization-Engineered Ga-Face GaN-Based Heterostructures for Normally-Off Heterostructure Field-Effect Transistors; J. of Electronic Materials, Vol. 42, No. 1 2013, to manufacture a field effect transistor comprising five different semiconductor layers deposited one above another. Firstly, a back barrier layer comprising GaN is deposited on a substrate. That is followed by a buried layer comprising undoped AlGaN or p-doped GaN. A channel layer comprising GaN is produced thereon. A barrier layer composed of AlGaN forms the termination. An additional gate layer composed of GaN is situated below the metallic gate contact. If no electrical voltage is present at the gate electrode, the Fermi level is below the conduction band minimum. Consequently, the channel of the field effect transistor is insulating. Such a transistor is referred to as a normally off transistor in the following description. As a result of a positive voltage being applied to the gate electrode, the Fermi level shifts above the conduction band minimum, with the result that the transistor becomes conducting.
This known transistor has the disadvantage that the gate threshold voltage at which the current flow in the channel commences is as low as only 200 mV. This has the effect that circuit design including the known transistor becomes very complex since the gate voltage has to be controlled very accurately. Furthermore, the known transistor has the disadvantage that the charge carrier concentrations in the conduction band are very low, which results in a low current density in the channel.
WO 2011/023607 A1 discloses a power transistor on the basis of group III nitrides, which power transistor has fast switching frequencies and has a low channel resistance, such that this known transistor can provide a high output power. However, this known transistor has the disadvantage that the channel is conducting if no gate voltage is present. In order to switch the channel to high resistance and to block the current flow through the transistor, a negative gate voltage is required. Such a transistor is referred to as a normally on transistor in the description hereinafter. This transistor has the disadvantage that complex protective circuits are required in order to avoid destruction of the transistor or of assemblies connected thereto in the case of a fault.
Thus, it is an object of the invention to provide a field effect transistor which shows normally off operation and having the capability of switching any of high electrical power and/or high frequencies. Furthermore, it is an object of the invention to provide a normally on transistor having a higher threshold voltage.